Field of the Invention
The present invention relates generally to broadband light sources and, more specifically, to high peak power ultrashort IR lasers.
Description of the Prior Art
Optical fiber based short pulsed IR sources are typically based on either mode locked fiber lasers or amplification of short pulsed diode lasers. Typically, mode locked fiber lasers can produce pulses with pulse widths of ˜100 fs to 10's of ps while pulsed diode lasers can produce pulses of pulse widths of 10's of ps and higher. Currently, such sources are limited to <2 μm due to the availability of mode locked fiber lasers and diode lasers. For example, thulium mode locked fiber lasers have been demonstrated at wavelengths up to ˜2 μm (Sharp et al., “190-fs passively mode-locked thulium fiber laser with low threshold,” Optics Letters, 12, 881-83 (1996), the entire contents of which are incorporated herein by reference).
For generation of high peak power ultrashort pulses in fiber at wavelengths greater than 2 μm, fiber based frequency conversion schemes are typically used. Examples of such schemes include Raman scattering in normal dispersion fiber or soliton self frequency shift (SSFS) in anomalous dispersion fiber. Stimulated Raman scattering is the process whereby the optical beam interacts with the vibrational modes of the medium and is downshifted in frequency. Cascaded Raman processes result when the downshifted frequency optical beam is itself shifted one or more times by the Raman process. Soliton self frequency shift is a form of intrapulse Raman whereby the blue frequency components of the pulse pump the red frequency components resulting in a wavelength shifted pulse. Again, SSFS may occur a number of times, depending on input pulse intensity, nonlinearity of medium, interaction length, etc. In all cases, dispersion compensation is typically included to maintain the pulse width.
U.S. Pat. No. 7,519,253 to Islam teaches a pump source consisting of a short pulse laser diode with wavelength of shorter than 2.5 μm and pulse width of at least 100 picoseconds (ps) with one or more optical amplifiers chains and a nonlinear fiber with anomalous dispersion at the diode wavelength that modulates the diode through modulation instability to form ultrashort pulses. Some limitations with Islam's source are that it requires an initial seed pulse duration of greater than 100 ps and it needs an anomalous dispersion fiber. Nishizawa demonstrated SSFS of a short pulsed ˜110 femtosecond (fs) 1.5 μm source out to ˜2.033 μm in nonlinear fiber (Nishizawa et al., “Widely wavelength-tunable ultrashort pulse generation using polarization maintaining optical fiber,” IEEE Journal on Selected Topics in Quantum Electronics, 7, 518-24 (2001)). Some limitations with Nishizawa's source is that it is lower power, has an initial seed pulse of less than 1 ps, and has a maximum wavelength of 2 μm. Imeshev took Nishizawa's work further and demonstrated that these pulses could be amplified in a thulium fiber amplifier, demonstrating amplification of a ˜400 fs 1.5 μm pulse train wavelength shifted in a nonlinear fiber and amplified to 1.98 μm in a thulium fiber amplifier (Imeshev et al., “230-kW peak power femtosecond pulses from a high power tunable source based on amplification in Tm-doped fiber,” Optics Express, 13, 7424-31 (2005)). Some limitations with Imeshev's source are that it has an initial seed pulse of less than 1 ps and it has a maximum wavelength of 2 μm. U.S. Patent Application 2010/0079853 by Rakich teaches a short pulsed source with low spontaneous noise component and with pulse shape that is optically flat which is Raman shifted to longer wavelengths through a cascaded Raman process in normal dispersion fibers. Some limitations of Rakich are that it needs an initial seed pulse duration of greater than 100 ps, it requires low amplified spontaneous emission in the fiber chain, and it does not give high power.
What is needed but not present in the prior art is a fiber based method of generating high peak power ultrashort (<1 ps) IR pulses at wavelength of >2 μm.